Twelve strains of oral methanogenic bacteria were isolated from supra- or subgingival plaque deposits by continuous culture techniques which yielded populations containing greater than 50% methanogens. Taxonomic studies based on substrate utilization studies, (e.g., C02+H2), formate and ethanol, morphological studies (cell wall composition) and DNA homology studies suggested that all twelve isolates belong to the genus Methanobrevibacter. However, interspecies comparisons between Methanobrevibacter smithii, methanobrevibacter ruminantion and the twelve isolates revealed that the oral strains were not related (at the species level) to any of the known intestinal or rumen Methanobrevibacter species. Furthermore, comparisons of DNA homologies among the twelve isolates indicated that they could be assigned to three distinct toxous. Thus, this initial study suggests that, at least, three distinct species of methanogenic bacteria exist in the oral cavity. Model food chain systems were demonstrated first in binary cultures in which H2 and C02 were supplied to one of the methanogenic isolates by Veillonella rodentium growing at the expense of lactate in Peptostreptococcus anaerobius growing at the expense of glucose and various amino acids. Tripartite cultures consisting of saccharolytic bacterium (i.e., Streptococcus mutans, Streptococcus sanguis or Lactobacillus casei). V. rodentium and one of the methanogenic isolates provide a functional food chain model wherein sugar was converted to lactate for formate ethanol and acetate which was in turn converted to substrates for the metahnogens (i.e., H2 and CO2). In this model, however, formate also served as a source for methane production with some of the metanogenic bacteria. The fact that methane production in the tripartile cultures was not as great as that observed with plaque samples suggests that the natural system provides a more efficient hydrogen transfer chain.